1. Field of the Invention
The present invention relates to a technique of reducing a motion artifact in executing tissue harmonic imaging (THI) using a difference tone.
2. Description of the Related Art
An ultrasonic diagnostic apparatus is a diagnostic apparatus for displaying an image of in vivo information. The ultrasonic diagnostic apparatus is less expensive and causes no exposure, as compared to other image diagnostic apparatuses such as an X-ray diagnostic apparatus or an X-ray computerized topographic apparatus, and is used as a useful apparatus for noninvasive observation in real time. The ultrasonic diagnostic apparatus has a wide application range because of its characteristics and is used to diagnose circulatory organs such as a heart, abdominal organs such as a liver and kidney, peripheral vessels, cerebral blood vessels or in diagnosis in obstetrics and gynecology.
Recently, various visualization methods using an ultrasonic diagnostic apparatus have been proposed. Typical examples are as follows. In a pulse inversion method, ultrasonic waves are transmitted in two rates with different phases, and obtained echo signals are added to remove the fundamental wave (Iwao Abiru & Tomoo Kamakura, “Nonlinear Propagation of Ultrasonic Pulse”, IEICF Technical Report US89-23, p. 53). In another method, one transmission wave is transmitted by controlling a plurality of frequencies and phases, and a harmonic component generated in a frequency band twice the fundamental wave, the difference tone component of the fundamental wave, and the chord component of the fundamental wave are visualized (e.g., Japanese Patent Application No. 2003-070935). In still another method, at least one rate in which the transmission pulse is turned off is inserted after two kinds of ultrasonic pulses obtained by inverting the phase by pulse inversion, and a plurality of reception signals are selectively added for each scan line, thereby reducing superimposition of a residual ultrasonic wave on the echo signal to be used for visualization.
In the above-described THI using a difference tone, the frequency of the difference tone overlaps the fundamental wave frequency. Hence, the difference tone component cannot properly be visualized. In this case, the fundamental wave component can be removed from the difference tone component by using pulse inversion. With this method, excellent diagnostic images can be collected for a part without any motion.
However, when tissue with quick motion (e.g., a heart) is visualized by THI using the difference tone, a motion artifact may be generated by the fundamental wave component. Although the fundamental wave component can be reduced from the difference tone component by using pulse inversion, the motion artifact cannot properly be reduced. When a normal filter is used, the motion artifact does not appear. However, it is impossible to properly remove the fundamental wave component superimposed on the difference tone component.